U.S. patent number 6,939,522 [Application Number 09/889,375] was granted by the patent office on 2005-09-06 for honeycomb structure.
This patent grant is currently assigned to NGK Insulators, Ltd.. Invention is credited to Takashi Harada, Yukio Miyairi.
United States Patent |
6,939,522 |
Harada , et al. |
September 6, 2005 |
Honeycomb structure
Abstract
A honeycomb structure (10) having a large number of
through-holes (11) formed in the axial direction and defined by
partition walls, wherein slits (12) are formed so as to be exposed
to at least part of the outer surface (13) of the honeycomb
structure along the axial direction. In this honeycomb structure,
each portion can deform freely without being restricted by other
portion even when an uneven temperature distribution appears
therein; as a result, reduction in thermal stress is possible and
generation of cracks can be prevented.
Inventors: |
Harada; Takashi (Nagoya,
JP), Miyairi; Yukio (Nagoya, JP) |
Assignee: |
NGK Insulators, Ltd.
(JP)
|
Family
ID: |
18225919 |
Appl.
No.: |
09/889,375 |
Filed: |
July 16, 2001 |
PCT
Filed: |
November 15, 2000 |
PCT No.: |
PCT/JP00/08044 |
371(c)(1),(2),(4) Date: |
July 16, 2001 |
PCT
Pub. No.: |
WO01/37971 |
PCT
Pub. Date: |
May 31, 2001 |
Foreign Application Priority Data
|
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|
|
|
Nov 19, 1999 [JP] |
|
|
11-329848 |
|
Current U.S.
Class: |
422/211 |
Current CPC
Class: |
F01N
3/2828 (20130101); F01N 3/0222 (20130101); B01D
39/2075 (20130101); Y02T 10/12 (20130101); F01N
2260/10 (20130101); Y02T 10/20 (20130101); B01J
35/04 (20130101) |
Current International
Class: |
B01D
39/20 (20060101); F01N 3/28 (20060101); F01N
3/022 (20060101); B01J 35/00 (20060101); B01J
35/04 (20060101); B01J 035/02 () |
Field of
Search: |
;422/211,174,175,177,179,180 ;428/116 ;55/DIG.30 ;60/300 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 283 220 |
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Sep 1988 |
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EP |
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0 787 524 |
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Aug 1997 |
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EP |
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50-114409 |
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Sep 1975 |
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JP |
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59-199586 |
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Nov 1984 |
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JP |
|
03-121218 |
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May 1991 |
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JP |
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3-258347 |
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Nov 1991 |
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JP |
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5-27215 |
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Apr 1993 |
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JP |
|
7-189666 |
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Jul 1995 |
|
JP |
|
07-189668 |
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Jul 1997 |
|
JP |
|
2000-153117 |
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Jun 2000 |
|
JP |
|
Primary Examiner: Johnson; Jonathan
Attorney, Agent or Firm: Parkhurst & Wendel, L.L.P.
Claims
What is claimed is:
1. A honeycomb structure having a large number of through-holes
formed in the axial direction and defined by partition walls,
wherein (1) slits are formed so as to be exposed to at least part
of the outer surface of the honeycomb structure along the axial
direction, (2) the honeycomb structure is made of a material
containing, as a main crystal phase, at least one member selected
from the group consisting of cordierite, SiC, SiN, alumina,
mullite, aluminum titanate and lithium aluminum silicate, and (3)
in each section of the honeycomb structure including each slit,
each slit is exposed along an upper end surface and a lower end
surface of the honeycomb structure and there is a continuous area
in the center of the honeycomb structure in which no slit is formed
and which is not exposed to the outer surface of the honeycomb
structure.
2. A honeycomb structure according to claim 1, wherein the slits
are formed in parallel to the direction of the through-holes.
3. A honeycomb structure according to claim 1, wherein the slits
are formed at least at one end surface at least at the edge.
4. A honeycomb structure according to claim 3, wherein the length
of each slit formed at the edge of one end surface is, in the axial
direction of the outer surface, 10% or more of the total length of
the honeycomb structure and, at the end surface, 10% or more of the
diameter of the honeycomb structure.
5. A honeycomb structure according to claim 1, wherein each slit is
exposed to at least one end surface so as to connect the two points
of the end surface edge.
6. A honeycomb structure according to claim 5, wherein the length
of each slit is, in the axial direction of the outer surface, 10%
or more of the total length of the honeycomb structure and, at the
end surface, 10% or more of the diameter of the honeycomb
structure.
7. A honeycomb structure according to claim 1, wherein each slit is
exposed to the outer surface over its total length in the
through-hole direction.
8. A honeycomb structure according to claim 1, wherein in the
honeycomb structure section which is normal to the through-holes
and in which the length of each slit is largest, the length of each
slit is 10% or more of the distance between the outer surface along
the axial direction and the center of the honeycomb structure.
9. A honeycomb structure according to claim 1, wherein in the
honeycomb section which is normal to the through-holes and in which
the length of each slit is largest, the length of each slit is 30%
or more of the distance between the outer surface along the axial
direction and the center of the honeycomb structure.
10. A honeycomb structure according to claim 1, wherein each slit
is filled with a filler.
11. A honeycomb structure according to claim 1, which is a
combination of two or more honeycomb segments.
12. A honeycomb structure according to claim 1, which loads thereon
a metal having a catalytic action and is usable for purification of
the exhaust gas emitted from a heat engine or a burner, or for
reforming of a liquid fuel or a gaseous fuel.
13. A honeycomb structure according to claim 12, wherein the metal
having a catalytic action is at least one kind selected from Pt, Pd
and Rh.
14. A honeycomb structure according to claim 1, wherein the
sectional shape of the through-holes is any of a triangle, a
tetragon, a hexagon and a corrugation.
15. A honeycomb structure according to claim 1, wherein the
partition walls surrounding the through-holes have a filtration
ability, a given proportion of the through-holes are blocked at one
end and the remaining through-holes are blocked at other end.
16. A honeycomb structure according to claim 15, which is used as a
filter for capturing and removing the particulate matter contained
in a dust-containing fluid.
Description
TECHNICAL FIELD
The present invention relates to a honeycomb structure for catalyst
loading, used in an exhaust gas purifier of a heat engine (e.g. an
internal combustion engine) or a burner (e.g. a boiler), a
reforming unit of a liquid fuel or a gaseous fuel, and the like; as
well as to a honeycomb structure used as a filter.
BACKGROUND ART
Honeycomb structures having a catalyst component loaded thereon
have been used in an exhaust gas purifier of a heat engine (e.g. an
internal combustion engine) or a burner (e.g. a boiler), a
reforming unit of a liquid fuel or a gaseous fuel, and the like. It
is known that honeycomb structures are also used as a filter for
capturing and removing the particulate matter contained in a
dust-containing fluid such as exhaust gas emitted from a diesel
engine.
The honeycomb structures used for such purposes have had problems;
for example, they undergo rapid temperature change or local heating
by the action of an exhaust gas, an uneven temperature distribution
appears therein, resultantly they come to have cracks. Particularly
when they were used as a filter for capturing the particulate
matter contained in the exhaust gas emitted from a diesel engine,
cracks appeared easily because the carbon fine particles
accumulated on the filter must be burnt for removal and it
inevitably causes local heating to high temperatures and easily
generates a large thermal stress. In this case, the thermal stress
is generated because the uneven temperature distribution allows
different portions of the honeycomb structure to have different
thermal expansion deformations and resultantly each portion is
restricted by each other and is unable to make free
deformation.
To reduce the stress, there was proposed, in, for example,
JP-A-59-199586, a honeycomb structure having a large number of
through-holes each surrounded by partition walls, wherein partition
wall areas having at least one slit are formed at given portions of
the honeycomb structure almost uniformly.
In this proposal, small slits are distributed uniformly in a
honeycomb structure, thereby the rigidity of the total honeycomb
structure is reduced and the freedom of deformation is increased,
therefore an effect of stress reduction is achieved. However, the
degree of an increase in the freedom of deformation is
insufficient; therefore, the above proposal for stress reduction
was insufficient for a honeycomb structure used under severe
conditions where the unevenness of temperature distribution becomes
larger.
In view of the above-mentioned problems of the prior art, the
present invention aims at providing a honeycomb structure which
gives no cracks by thermal stress during use and which is superior
in durability.
DISCLOSURE OF THE INVENTION
According to the present invention there is provided a honeycomb
structure having a large number of through-holes formed in the
axial direction and defined by partition walls, wherein slits are
formed so as to be exposed to at least part of the outer surface of
the honeycomb structure along the axial direction.
In the above honeycomb structure, the slits are formed preferably
in parallel to the direction of the through-holes. Also,
preferably, the slits are formed at least at one end surface at
least at the edge. By forming the slits in this way, each portion
of the honeycomb structure can deform freely without being
restricted by other portion even when an uneven temperature
distribution appears therein; as a result, reduction in thermal
stress is possible and generation of cracks can be prevented.
In this case, more preferably, the length of each slit formed at
the edge of one end surface is, in the axial direction of the outer
surface, 10% or more of the total length of the honeycomb structure
and, at the end surface, 10% or more of the diameter of the
honeycomb structure.
It is also preferable that each slit is exposed to at least one end
surface so as to connect the two points of the end surface edge,
because such a honeycomb structure has an increased freedom for
deformation in the vicinity of the end surface, reduction in
thermal stress is possible, and generation of cracks is prevented.
In this case, more preferably, the length of each slit is, in the
axial direction of the outer surface, 10% or more of the total
length of the honeycomb structure and, at the end surface, 10% or
more of the diameter of the honeycomb structure.
When the present honeycomb structure is used under such conditions
that the total (total length) of the honeycomb structure comes to
have a temperature unevenness, it is preferable that each slit is
exposed to the outer surface over its total length in the
through-hole direction.
When the total of the honeycomb structure comes to have a large
temperature unevenness, it is particularly preferable that each
slit is formed so that in each section of the honeycomb structure
including each slit there is a continuous area in which no slit is
formed and which is not exposed to the outer surface of the
honeycomb structure.
In the honeycomb structure of the present invention, it is
preferable that in the honeycomb structure section which is normal
to the through-holes and in which the length of each slit is
largest, the length of each slit is 10% or more, preferably 30% or
more of the distance between the outer surface along the axial
direction and the center of the honeycomb structure.
In the honeycomb structure of the present invention, it is
preferable that each slit is filled with a filler. It is also
preferable that the present honeycomb structure is a combination of
two or more honeycomb segments.
The above honeycomb structure is preferably made of a material
containing, as a main crystal phase, at least one kind selected
from the group consisting of cordierite, SiC, SiN, alumina,
mullite, aluminum titanate and lithium aluminum silicate.
Preferably, the present honeycomb structure loads thereon a metal
having a catalytic action and is usable for purification of the
exhaust gas emitted from a heat engine (e.g. an internal combustion
engine), a burner (e.g. a boiler) or the like, or for reforming of
a liquid fuel or a gaseous fuel. The metal having a catalytic
action is preferably at least one kind selected from Pt, Pd and
Rh.
In the present honeycomb structure, the sectional shape of the
through-holes is preferably any of a triangle, a tetragon, a
hexagon and a corrugation from the standpoint of the production of
the honeycomb structure.
When the present honeycomb structure is used as a filter for
capturing and removing the particulate matter contained in a
dust-containing fluid, for example, as a particulate filter for
diesel engine, it is preferable that the partition walls
surrounding the through-holes have a filtration ability, a given
proportion of the through-holes are blocked at one end and the
remaining through-holes are blocked at other end.
In the present invention, "the outer surface of the honeycomb
structure" refers to the whole outer surface of the honeycomb
structure and includes not only the outer surface along the axial
direction but also the end surfaces.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is perspective views showing examples of the honeycomb
structure according to the present invention.
FIG. 2 is perspective views showing other examples of the honeycomb
structure according to the present invention.
FIG. 3 is perspective views showing still other examples of the
honeycomb structure according to the present invention.
FIG. 4 is perspective views showing still other examples of the
honeycomb structure according to the present invention.
FIG. 5 is perspective views showing still other examples of the
honeycomb structure according to the present invention.
FIG. 6(a) is a perspective view showing still other example of the
honeycomb structure according to the present invention. FIG. 6(b)
is a view taken along the Y--Y section of FIG. 6(a).
FIG. 7(b) is a perspective view showing still other example of the
honeycomb structure according to the present invention. FIG. 7(b)
is a view taken along the Z--Z section of FIG. 7(a).
FIG. 8(a) shows a method for slit formation and indicates a state
in which slits are formed in parallel to partition walls. FIG. 8(b)
indicates a state in which slits are formed so as to cut partition
walls obliquely.
FIG. 9(a) shows a stress-relaxing mechanism provided at the front
end of each slit, i.e. a stress-relaxing portion having a curvature
provided at the front end of each slit. FIG. 9(b) shows branches
provided at the front end of each slit.
FIG. 10(a) shows a form of slit formed by cutting part of partition
walls. FIG. 10(b) shows a form of slit formed by removing part of
partition walls.
FIG. 11(a) is a plan view showing an example of the disposition of
slits in the honeycomb structure of the present invention; FIG.
11(b) is a front view of the example; FIG. 11(c) is a side view of
the example; and FIG. 11(d) is a bottom view of the example.
BEST MODE FOR CARRYING OUT THE INVENTION
The honeycomb structure of the present invention has a large number
of through-holes formed in the axial direction and defined by
partition walls, wherein slits are formed so as to be exposed to at
least part of the outer surface of the honeycomb structure.
Therefore, the present honeycomb structure generates no cracks by
the thermal stress appearing during the use and is extremely
superior in durability.
The present invention is described in more detail below with
reference to the embodiments shown in the accompanying drawings.
However, the present invention is in no way restricted to these
embodiments.
FIGS. 1(a), (b), (c) and (d) are perspective views showing
embodiments of the honeycomb structure of the present
invention.
In FIGS. 1(a), (b), (c) and (d), 10 is a honeycomb structure. The
honeycomb structure 10 has a large number of through-holes 11
formed in the axial direction and defined by partition walls. In
the honeycomb structure 10, four slits 12 are formed so as to be
exposed to part of the outer surface 13 along the axial direction
and be parallel to the direction of the through-holes 11. Each slit
12 is formed so as to cross the edge 15 of the end surface 14.
In FIG. 1(a), four slits 12 are formed so as to cross the edge 15
of the end surface 14 and each in the shape of a triangle. In FIG.
1(b), each slit 12 is formed in the shape of a tetragon. In FIG.
1(c), each slit 12 is formed so as to be exposed to the axial
direction total length of the outer surface 13 along the axial
direction of the honeycomb structure 10 and so that the depth of
the slit 12 becomes gradually smaller and each slit has a
triangular shape. In FIG. 1(d), each slit 12 is formed so as to be
exposed to the axial direction total length of the outer surface 13
along the axial direction of the honeycomb structure 10 and so that
the depth of the slit 12 becomes constant in the axial direction of
the honeycomb structure 10. By forming slits 12 as shown in FIGS.
1(a), (b), (c) and (d), each portion of the honeycomb structure 10
can deform freely without being restricted by other portion even
when an uneven temperature distribution appears in the honeycomb
structure 10 (that is, temperature difference exists between the
portions of the honeycomb structure 10); thereby, the thermal
stress is reduced; and generation of cracks is minimized.
In FIG. 2(a), three slits 12 are formed. As in FIG. 1(c), each slit
12 is formed so as to be exposed to the axial direction total
length of the outer surface 13 along the axial direction of the
honeycomb structure 10 and so that the depth of the slit 12 becomes
gradually smaller and each slit has a triangular shape. In FIG.
2(b), three slits 12 are formed and, as in FIG. 1(d), each slit 12
is formed so as to be exposed to the axial direction total length
of the outer surface 13 along the axial direction of the honeycomb
structure 10 and so that the depth of the slit 12 becomes constant
in the axial direction of the honeycomb structure 10. These
embodiments are particularly effective when the honeycomb structure
is used under such conditions that an uneven temperature
distribution appears over the total length of the honeycomb
structure.
FIGS. 3(a) and (b) are perspective views showing other examples of
the honeycomb structure of the present invention.
In the example of FIG. 3(a), slits 12 are formed so as to be
exposed to one end surface 14 and connect the two points (A and B)
or (C and D) of an end surface edge 15. In the example of FIG.
3(b), slits 12 are formed at two end surfaces 14 and 16 so as to be
exposed to each of them and connect each two points of an end
surface edge 15.
FIGS. 4(a), (b), (c) and (d) are perspective views showing still
other examples of the honeycomb structure of the present
invention.
In each example of FIGS. 4(a), (b), (c) and (d), in each section of
a honeycomb structure 10 including a slit 12 there is a continuous
area 17 having no slit 12 formed, in the center of the honeycomb
structure 10. This continuous area 17 is not exposed to the outer
surface of the honeycomb structure, that is, any of the outer
surface 13 along the axial direction, the upper end surface 14 and
the lower end surface 16. Incidentally, the shape of the continuous
area 17 is a rectangle in FIG. 4(a), a circle in FIG. 4(b), a
racetrack in FIG. 4(c) and an equilateral tetragon in FIG. 4(d). By
forming a honeycomb structure in such a constitution, no cracks are
generated in the honeycomb structure even when a large temperature
unevenness (for example, large temperature difference between the
portions of the honeycomb structure) appears in the whole honeycomb
structure.
In each example of FIGS. 5(a), (b), (c) and (d), in each section of
a honeycomb structure 10 including a slit 12 there is a continuous
area 17 having no slit 12 formed and part of the continuous area 17
is exposed to the lower end surface 16 of the honeycomb structure
10.
FIGS. 6(a) and (b) and FIGS. 7(a) and (b) are other examples in
which a continuous area is not exposed to the outer surface of a
honeycomb structure. FIG. 6(a) is a perspective view and FIG. 6(b)
is a sectional view taken along the Y--Y section of FIG. 6(a). FIG.
7(a) is a perspective view and FIG. 7(b) is a sectional view taken
along the Z--Z section of FIG. 7(a).
In the example of FIGS. 6(a) and (b), the shape of a continuous
area 17 is a rectangle as in FIG. 4(a), and the continues area 17
is not exposed to the outer surface of the honeycomb structure 10.
In this example, however, the number of slits 12 is larger than in
FIG. 4(a). In the example of FIG. 7(a) and (b), the shape of a
continuous area 17 is an ellipse, and the continuos area 17 is not
exposed to the outer surface of the honeycomb structure 10.
Next, the constituent features of the honeycomb structure of the
present invention are described in more detail.
In the honeycomb structure of the present invention, the length of
each slit is preferably 10% or more, more preferably 30% or more of
the distance between the outer surface along the axial direction
and the center of the honeycomb structure, in a honeycomb structure
section which is normal to the through-holes and in which the
length of the slit becomes largest.
In the honeycomb structure of the present invention, slits are
preferably formed in point symmetry in a honeycomb structure
section normal to the through-holes because deformation occurs in
the honeycomb structure with substantially no partiality. However,
slit formation is not restricted thereto and slits 12 may be
disposed, for example, as shown in FIGS. 11(a) to (d).
Slits 12 may be formed so as to cut partition walls 20 obliquely
(not in parallel thereto), as shown in FIG. 8(b). However, slits
are preferably formed in parallel to partition walls 20, as shown
in FIG. 8(a), because the stress concentration at the front end of
each slit 12 is small.
When the sectional shape of the through-holes 11 of the honeycomb
structure 10 is a triangle, slits 12 are preferably in a direction
of 60.degree. or 120.degree., for the same reason as mentioned
above.
The width of each slit 12 is not critical and can be determined as
desired. However, the width is desirably smaller than the width of
each through-hole because too large a slit width incurs increases
in filling steps and filling amount of filler and a decrease in the
number of cells usable for purification of fluid (e.g. gas).
Further in the honeycomb structure 10 of the present invention, it
is preferable for relaxation of thermal stress to form, at the
front end of each slit 12, a branch part 21 formed by branching, as
shown in FIG. 9(b), or a stress relaxation part 22 having a
curvature, as shown in FIG. 9(a).
The form of each slit 12 may be a form obtained by partially
cutting partition walls 20 of a honeycomb structure 10, as shown in
FIG. 10(a), or a form obtained by partially removing partition
walls 20, as shown in FIG. 10(b).
In the honeycomb structure of the present invention, the sectional
shape of each through-hole may be various, for example, a circle,
an ellipse and a racetrack.
The honeycomb structure of the present invention is preferably a
combination of two or more honeycomb segments. The material
constituting the present honeycomb structure preferably contains,
as a main crystal phase, at least one kind selected from the group
consisting of cordierite, SiC, SiN, alumina, mullite, aluminum
titanate and lithium aluminum silicate. SiC of high thermal
conductivity is particularly preferred because it releases the
absorbed heat easily.
The density of the cells surrounded by partition walls is
preferably 6 to 2,000 cells/in..sup.2 (0.9 to 311 cells/cm.sup.2),
more preferably 50 to 400 cells/in..sup.2 (7.8 to 62
cells/cm.sup.2). When the cell density is less than 6
cells/in..sup.2 (0.9 cell/cm.sup.2), the resulting honeycomb
segments are insufficient in strength and effective GSA
(geometrical surface area); when the cell density is more than
2,000 cell/in..sup.2 (311 cells/cm.sup.2), the resulting honeycomb
segments show a large pressure loss when a gas flows
therethrough.
The sectional shape of the through-holes, i.e. the cell shape is
preferably any of a triangle, a tetragon and a hexagon from the
standpoint of honeycomb production.
With respect to the filler used for filling the slits formed in the
honeycomb structure, a ceramic fiber, a ceramic powder, a cement,
etc. all having heat resistance are preferably used singly or in
admixture. As necessary, an organic binder, an inorganic binder,
etc. may be mixed with the filler.
When the honeycomb structure of the present invention is used as a
catalyst carrier for purification of the exhaust gas emitted from a
heat engine (e.g. an internal combustion engine) or a burner (e.g.
a boiler), or for reforming of a liquid fuel or a gaseous fuel, a
metal having a catalytic activity is loaded on the honeycomb
segments constituting the honeycomb structure. As representative
metals having a catalytic activity, there can be mentioned Pt, Pd
and Rh. At least one kind of these metals is preferably loaded on
the honeycomb segments.
Meanwhile, when the honeycomb structure of the present invention is
used as a filter for capturing and removing the particulate matter
contained in a dust-containing fluid, for example, a particulate
filter for diesel engine, the honeycomb structure preferably has
such a structure that the partition walls surrounding the
through-holes has a filtration ability, given proportions of the
through-holes are blocked at one end, and the remaining
through-holes are blocked at other end.
When a dust-containing fluid is allowed to enter such a honeycomb
structure from one end surface, the dust-containing fluid enters
the inside of the honeycomb structure from those through-holes not
blocked at the above end surface, passes through the partition
walls having a filtration ability, and enters other through-holes
not blocked at other end surface. During the period in which the
dust-containing fluid passes through the partition walls, the
particulate matter contained in the fluid is captured by the
partition walls, and a particulate matter-free, purified fluid is
discharged from the other end surface of the honeycomb
structure.
As the accumulation of the captured particulate matter on the
partition walls proceeds, plugging appears, which reduces the
filtration ability of the honeycomb structure. Therefore, the
honeycomb structure is periodically heated using a means such as
heater or the like, whereby the captured particulate matter are
burnt completely and the filtration ability is revitalized. In
order to promote the burning of the particulate matter during the
revitalization, it is possible to allow the honeycomb structure to
load the above-mentioned metal having a catalytic action.
The present invention is described in more detail below by way of
Examples. However, the present invention is not restricted to these
Examples.
EXAMPLE 1
By using a SiC-made honeycomb structure having a size of 144 mm
(diameter) and 152 mm, a partition wall thickness of 0.3 mm and a
cell density of 31 cells/cm.sup.2 and by blocking given proportions
of the through-holes at one end and the remaining through-holes at
other end, there were produced honeycomb structures each as a
particulate filter for purification of diesel engine exhaust gas.
They were produced so as to have slits as shown in FIG. 1(a), FIG.
1(d) FIG. 3(a), FIG. 3(b), FIG. 4(a), FIG. 4(b) or FIG. 7, or so as
to have no slit (a basic honeycomb structure). In the honeycomb
structures of FIGS. 1(a) and 1(d), the length of each slit at the
upper end surface 14 was 1/2 of the radius of the honeycomb
structure 10. In the honeycomb structures of FIGS. 1(a) and 3(a),
the axial direction length of each slit at the outer surface 13 was
20 mm; in the honeycomb structure of FIG. 3(b), the axial direction
outer surface length of each slit formed at the upper end surface
14 was 15 mm and the axial direction outer surface length of each
slit formed at the lower end surface 16 was 30 mm.
In the thus-produced honeycomb structures, those cells in which
slits were formed, were sealed with a filler; then, a ceramic-made
non-expansion mat (as a holding member) was wound round the
periphery of each honeycomb structure; the resulting honeycomb
structure was forced into a SUS 409-made canning case to produce
various canning structures. These canning structures were each
subjected to the following filter revitalization test. That is, a
soot-containing combustion gas generated by burning of a gas oil
for diesel engine was allowed to flow into each honeycomb structure
from the lower end surface (the lower end surface in each Drawing)
and leave from the upper end surface, whereby the soot in the
combustion gas was captured inside the honeycomb structure. The
soot-captured honeycomb structure was allowed to cool to room
temperature, after which a combustion gas containing a given
proportion of oxygen was allowed to flow into the honeycomb
structure from the lower end surface at 800.degree. C. to
completely burn the soot captured in the honeycomb structure.
The transition period necessary for elevating the inlet gas
temperature to 800.degree. C. and the weight of the soot captured
were each set at three levels, and the filter revitalization test
was carried out. The appearance of cracks was examined at each site
of the upper end surface (outlet), lower end surface (inlet), outer
surface along axial direction and inside of each honeycomb
structure. The results are shown in Table 1.
TABLE 1 Soot weight Largest Large Standard Transition period
Shortest Short Standard Outer Outer Outer Site of cracks Inlet
Outlet Inside surface Inlet Outlet Inside surface Inlet Outlet
Inside surface FIG. 1(a) .tangle-solidup. .tangle-solidup.
.tangle-solidup. .tangle-solidup. .tangle-solidup. .largecircle.
.largecircle. .tangle-solidup. .largecircle. .largecircle.
.largecircle. .largecircle. FIG. 1(d) .tangle-solidup.
.tangle-solidup. .tangle-solidup. .tangle-solidup. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. FIG. 3(a)
.tangle-solidup. .largecircle. .tangle-solidup. .tangle-solidup.
.tangle-solidup. .largecircle. .tangle-solidup. .tangle-solidup.
.largecircle. .largecircle. .largecircle. .largecircle. FIG. 3(b)
.largecircle. .largecircle. .tangle-solidup. .tangle-solidup.
.largecircle. .largecircle. .tangle-solidup. .tangle-solidup.
.largecircle. .largecircle. .largecircle. .largecircle. FIG. 4(a)
.largecircle. .largecircle. .tangle-solidup. .tangle-solidup.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle. FIG. 4(b)
.largecircle. .largecircle. .largecircle. .tangle-solidup.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle. FIG. 7
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle. No slit
.tangle-solidup. .tangle-solidup. .tangle-solidup. .tangle-solidup.
.tangle-solidup. .tangle-solidup. .tangle-solidup. .tangle-solidup.
.tangle-solidup. .tangle-solidup. .largecircle. .largecircle.
.tangle-solidup.: Cracks generated. .largecircle.: No cracks.
As is clear from Table 1, even in the case of the standard
conditions, cracks appeared in the honeycomb structure having no
slit, at the end surfaces (inlet and outlet). In contrast, no
cracks appeared in the honeycomb structures of the present
invention of FIG. 1(a), FIG. 1(d), FIG. 3(a), FIG. 3(b), FIG. 4(a),
FIG. 4(b) and FIG. 7.
When the transition period was shortened and the weight of the soot
captured was increased, temperature unevenness became larger and
cracks appeared partially when slits were formed only in the
vicinity of one end surface of honeycomb structure as in FIG. 1(a)
or FIG. 3(a); however, substantially no cracks appeared when a
continuous area was not exposed to the outer surface of honeycomb
structure as in FIG. 4(b), and no cracks appeared when slits were
formed in an increased number as in FIG. 7.
EXAMPLE 2
Using honeycomb structures shown in Table 2, having the same shapes
and sizes as in Example 1 except that the dispositions and lengths
of slits were changed, a filter revitalization test of completely
burning and removing the soot accumulated was carried out in the
same manner as in Example 1. The results are shown in Table 2.
TABLE 2 Sample No. 1 2 3 4 Structure FIG. FIG. FIG. FIG. 1(a) 3(a)
1(a) 3(a) Slit length at end 50 mm 15 mm 15 mm 5 mm surface Slit
length at 30 mm 15 mm 5 mm 5 mm outer surface Weight of soot
captured 10 g/L .largecircle. .largecircle. .largecircle.
.largecircle. 12 g/L .largecircle. .largecircle. .largecircle. X 14
g/L .largecircle. .largecircle. X X 16 g/L .largecircle.
.largecircle. X X 18 g/L .largecircle. .largecircle. X X 20 g/L
.largecircle. X X X .largecircle.: No cracks. X: Cracks appeared.
g/L: gram/liter.
As is clear from the results of Table 2, an increase in weight of
soot captured results in an increase in cracks formed when the
lengths of slit at end surface and outer surface are smaller than
given levles.
A described above, in the honeycomb structure of the present
invention, each portion of the honeycomb structure can deform
freely without being restricted by other portion even when there is
an uneven temperature distribution in the honeycomb structure and,
therefore, reduction in thermal stress is possible; as a result,
generation of cracks can be prevented.
* * * * *